The present disclosure relates generally to white light-shielding polymer compositions. In particular, it relates to polycarbonate compositions which have a white color, are opaque (i.e. light-shielding), and exhibit good mechanical properties. Also included are articles formed (i.e. molded, extruded, etc.) from such compositions.
Polycarbonates are synthetic thermoplastic resins derived from bisphenols and phosgenes, or their derivatives. They are linear polyesters of carbonic acid and can be formed from dihydroxy compounds and carbonate diesters, or by ester interchange. Polymerization may be in aqueous emulsion or in nonaqueous solution.
Polycarbonates have many properties and/or characteristics that are desired in certain instances. These include clarity or transparency (i.e. 90% light transmission or more), high impact strength, heat resistance, weather and ozone resistance, good ductility, being combustible but self-extinguishing, good electrical resistance, noncorrosive, nontoxic, etc. Furthermore, polycarbonates can be readily used in various article formation processes, such as molding (injection molding, etc.), extrusion, and thermoforming, among others. As a result, polycarbonates are used frequently to form a wide variety of products including: molded products, solution-cast or extruded films, structural parts, tubes and piping, lenses, safety shields, instrument windows, and medical devices. Household articles formed from polycarbonates can be produced in a great variety of colors and can be painted, glued, planed, pressed, and metalized and can be used to form precision parts and electronic products.
In view of some of the characteristics noted above, parts made from polycarbonate resins are continually being made thinner for various reasons. Among the reasons is a desire for consumer electronic products, such as cell phones, to be smaller and lighter. Polycarbonates are used, for example, to form the outside structure or housing of a cell phone.
One important property affected by the trend of thinner parts is opacity. Opacity is generally defined herein as the quality of not being penetrable by light. In this regard, the wall of a cell phone is opaque if light on one side of the wall cannot be seen on the other side of the wall. The property of opacity, or light-shielding, ensures that light only appears where it is desired. For example, the number keys of a cell phone should be lit, but not the space between the number keys. Opacity is measured by the percent transmission (% T) of light through an article having a defined thickness. The lower the % T, the more opaque the article is. Light-shielding is progressively more difficult to achieve as the part thickness decreases.
Other desired properties for forming thinner parts include good mechanical properties (e.g. impact strength at low temperatures), dimensional stability, and stability of the polymer during processing. When a dark-colored polycarbonate is acceptable for end use, high opacity or light-shielding is easily achieved by the use of dark colorants. However, when the polycarbonate must be white, it is more difficult to achieve high opacity, especially as the part thickness decreases. Whiteners such as titanium dioxide (TiO2) and zinc oxide (ZnO) are known to maintain a white color and increase opacity. However, high loadings, for example 15 weight percent to 20 weight percent TiO2 or more, are needed to create a product with a % T of less than 1% for part thicknesses of 1.0 millimeter or less. At these high loadings, properties such as stability of the polymer during processing and mechanical properties such as impact strength are adversely affected. In addition, these additives increase the weight of the finished article, which is also contrary to trends in the consumer electronics industry.
It would be desirable to provide light-shielding polycarbonate compositions which are of white color, opaque, have superior impact strength over a wide range of temperatures, especially at low temperatures of less than 0° C., and have favorable resin stability during processing of articles having wall thicknesses of 1 millimeter or less.